Buffer pool extension for database server

In a computing environment, a system, comprising:

• one or more processors;
• first tier memory, second tier memory that is slower than the first tier memory, and third tier memory that is slower than the second tier memory;
• and
• one or more computer-readable storage devices having stored thereon computer executable instructions, which when executed by the one or more processors perform actions, comprising:
• accessing metadata concerning a plurality of pages of a buffer pool, the buffer pool including one or more pages that are stored in the first tier memory, and one or more pages that are stored in the second tier memory;
• determining, from the metadata, a first threshold and a second threshold, wherein the first threshold and a second threshold are usable to classify the pages within one of at least three classifications, wherein:
• (i) a page is classified with a first classification if applying a function to access data represented by the metadata of the page returns a value less than the first threshold;
• (ii) the page is classified with a second classification if applying the function to access the data represented by the metadata of the page returns a value greater than the first threshold but less than the second threshold;
• or
• (iii) the page is classified with a third classification if applying the function to the access data of the metadata of the page returns a value greater than the second threshold;
• and based on the first and second thresholds:
• evicting a particular page from the second tier memory to the third tier memory, to free space for one or more other pages in the second tier memory, when the particular page is classified with the first classification;
• evicting the particular page from the first tier memory to the second tier memory, to free space for one or more other pages in the first tier memory, when the particular page is classified with the second classification;
• and
• leaving the particular page in the first tier when the particular page is classified with the third classification.

A hardware storage device having stored thereon computer-executable instructions, which when executed perform actions, comprising:

• receiving a request for data of a data set;
• determining that the data resides in secondary storage other than main memory and that is slower than the main memory;
• determining that a buffer pool in the main memory is full;
• selecting a candidate page for eviction from the buffer pool in main memory based on a function that classifies the candidate page according to one of at least three classifications according to accesses to the candidate page, wherein:
• (i) the candidate page is classified with a first classification if applying the function returns a value less than a first threshold;
• (ii) the candidate page is classified with a second classification if applying the function returns a value greater than the first threshold but less than a second threshold;
• or
• (iii) the candidate page is classified with a third classification if applying the function returns a value greater than the second threshold;
• and
• determining whether and how to evict the candidate page based on the one of at least three classifications, including:
• when the candidate page is classified with the first classification, determining that the candidate page should be evicted from the secondary storage to third storage that is slower than the secondary storage, to free space for one or more other pages in the secondary storage;
• when the candidate page is classified with the second classification, determining that the candidate page should be evicted from the main memory to the secondary storage, to free space for one or more other pages in the main memory;
• and
• when the candidate page is classified with the third of the classifications, determining that the candidate page should be left in the main memory.